1. Field of the Invention
This invention relates to phosphate conversion coatings for metal corrosion inhibition, particularly for substrates having two or more different metals whereby the substrate metals are simultaneously phosphated. The method of the invention is particularly useful for automotive body parts having joints of welded galvanized steel members where both steel (iron) and zinc are exposed. Good quality conversion coatings are obtained without "white spotting"; i.e. pitting of zinc substrates. Contemplated by the invention are the coated substrates themselves, solutions for coating them, and the methods for using these solutions.
2. Statement of Related Art
Phosphate conversion coatings for protecting corrodible metal surfaces and for providing a base for a paint or other finish coating are generally formed by treating a metal substrate wih a phosphating solution containing phosphoric acid and, usually, one or more additives such as oxidizing agents, acids, accelerators, or carriers.
The successful application of such phosphate conversion coatings to the metal substrate is dependent upon the reaction between the surface metal and the phosphating solution components; for example, in steel substrates, free iron (Fe.degree.) is oxidized and reacted to form the corresponding phosphate. While the reaction between iron and phosphoric acid theoretically proceeds with a concentrated solution of free phosphoric acid as sole reagent, it has been found that the resultant conversion coatings are not generally commercially acceptable. For example, oxidizing agents such as chlorate and/or nitrite ions are usually included in phosphating solutions for steel substrates to accelerate the interaction between steel and phosphoric acid in forming a crystalline phosphate coating on the steel surface.
In contrast, zinc surfaces such as galvanized steel are readily phosphated without the use of strong oxidizers, as the reaction of phosphoric acid with zinc to form the corresponding phosphate proceeds readily with hydrogen evolution. Not only are these phosphate coatings radily formed without the use of strong oxidizers, but the presence of oxidizers (particularly chlorates) in the phosphating solution generally causes pitting and etching of the substrate surface together with "white spot" (ZnO) formation. Even weak oxidizers such as nitrites, or free oxygen may cause pitting and white spots under high redox conditions, particularly at high free acidity.
Thus, phosphating solutions must be formulated with respect to the electrochemical characteristics of the metal substrate surface according to art-recognized principles. While these principles are readily applicable to the preparation of phosphating solutions for metal substrates having substantially uniform electrochemical surface characteristics, the formulation of phosphating solutions which simultaneously provide good-to-excellent coatings when two or more substances are present including at least one or more metal having different electrochemical surface characteristics, has not been entirely successful. Such multi-metal workpieces are frequently encountered in industry; e.g., automobile parts wherein two galvanized steel elements are joined, where electrogalvanized steel sheet metal is joined to non-galvanized cold rolled steel, where the workpiece has included impurities which may be other metals or inorganic materials, or where the workpiece is a discontinuous alloy. Substrates having more than one metal are conventioally phosphated under redox conditions sufficiently high to oxidize the substrate metal having the more negative redox potential, which frequently adversely affects the coating process for the substrate metal having the more positive redox potential. The problem is particularly associated with phosphating processes for substrates having steel and zinc surfaces as discussed above. Industry efforts to counteract zinc etching and white spot formation with reformulation baths having reduced amounts of carrier metal (nickel ions for example) have not heretofore produced the desired quality coatings. With increased use of galvanized steel for automobile parts routinely exposed to a corrosive environment, there is an industry need for a phosphating process which effectively will protect these parts and also provide a good base for finish coatings, particularly with respect to appearance, adhesion, coating weight, alkaline resistance and corrosion stability.
Prior art addressing this problem includes the disclosure of published Japanese patent application No. 81/108,682 filed June 24, 1982, and corresponding published Australian patent application No. 16,159/83, filed June 23, 1983, broadly relating to a phosphating solution including zinc, phosphate, and fluoride ions in specific concentrations useful on joined steel and galvanized steel substrates, such as encountered in contemporary automobile bodies.
U.S. Pat. No. 3,269,877 discloses a nickel-free phosphate coating composition which is limited in its disclosure to the coating of ferrous metal surfaces, and which includes cobalt ions.
Although not directed to the problem of phosphating two or more useful surfaces simultaneously, published PCT patent application Nos. WO84/00386 and WO85/03089 are of interest. These disclosures are directed to alkali resistant coatings which are made with primarily nickel-containing phosphating baths but do disclose nickel-free baths containing divalent metal cations whose hydroxides have a lower solubility in alkaline solution than iron or zinc, such as cobalt ions and other multivalent ions. The phosphate coating composition is critically controlled within a narrow range of components, so as to replace part of the zinc in the coating. The deposited coating contains at least 25% by weight of the coating of zinc and at least 15 mol percent of the total divalent cations in the coating comprise those metal cations whose solubility in alkali solution is less than that of zinc. Such coatings contain much higher than usual amounts of nonzinc metal in the coating.